A) Field of the Invention
The present invention relates to a magnetic tunneling junction element having an oxide film such as alumina (aluminum oxide) film.
B) Description of the Related Art
Magnetic tunneling junction elements are known as megnetoresistive elements to be used for magnetic heads, magnetic memories, magnetic sensors and the like. As a magnetic tunneling junction element manufacture method, a method as illustrated in FIGS. 10 to 12 is known (e.g., JP-A-2000-91668).
In the process illustrated in FIG. 10, on the surface of a ferromagnetic layer 1 made of Fe or the like, an aluminum film 2 of 2 nm in thickness is formed by sputtering. Next, pure oxygen is introduced into a sputtering chamber, and the aluminum layer 2 is oxidized for 10 minutes by setting an oxygen pressure in a range from 20 mTorr to 200 Torr. An alumina film 3 is therefore formed on the surface of the aluminum film 2 as shown in FIG. 11. This alumina film 3 is used as a tunneling barrier film. Thereafter, in the process illustrated in FIG. 12, a ferromagnetic layer 4 made of Co—Fe alloy or the like is formed on the alumina film 3 by sputtering.
As a method of forming an alumina film as a tunneling barrier film, other methods are also known, including (a) a method of exposing an aluminum film in the air to make it subject to natural or native oxidation and (b) a method of subjecting an aluminum film to a plasma oxidation process (for the method (a), refer to JP-2000-91668, and for the method (b), refer to JP-2000-36628).
With the conventional method (b), oxidation becomes likely to be too excessive so that an underlying ferromagnetic layer may be oxidized at the interface with the tunneling barrier film and the variation in a magnetic tunneling resistance may become lower.
With the conventional method (a), it takes a long time, several hours, to complete the oxidation process so that the tunneling barrier film may be formed with pin holes or contaminated by the presence of dusts in the air and the film quality may be degraded.
Although the method illustrated in FIGS. 10 to 12 is an improved method of the method (a), an aluminum film 2 not oxidized is likely to be left under the alumina film 3 as shown in FIG. 11. The left aluminum film 2 lowers the variation in the magnetic tunneling resistance. If oxidation of the aluminum film is insufficient, an electrostatic breakdown voltage of the magnetic tunneling junction lowers and a time-dependent change in the variation in the magnetic tunneling resistance becomes large when the magnetic tunneling junction element is placed in a high temperature environment. From these reasons, the reliability of a magnetic tunneling junction element lowers.